For Configuring Grandstream PBX with didforsale, you need to create four sip trunks. Two trunks for incoming calls and two trunks for outgoing.
For creating trunks, go to PBX => VoIP Trunks => Create new SIP Trunk. Add the details as shown in below figure
Similarly create three more sip trunks with the following IP address
Once all the trunks are added, the page will looks like
Now you need to configure your DID number.
For configuring DID number, go to PBX => Inbound Routes => Create New Inbound Rule and add the details as shown in below figure
Now configure outbound routes for sending outgoing calls through didforsale.
For outbound routes, go to PBX => outbound Routes => Create New Outbound Rule as shown in below figure
Select any one of the outgoing trunks in outbound routes. If you want to do a failover, click the option “click to add failover trunk” on the bottom side and select the second outbound trunk here.